This change begins to implement some of the AST custom decoding, beneath
the Package's Module map. In particular, we now unmarshal "one level"
beyond this, populating each Module's ModuleMember map. This includes
Classes, Exports, ModuleProperties, and ModuleMethods. The Class AST's
Members have been marked "custom", in addition to Block's Statements,
because they required kind-directed decoding. But Exports and
ModuleProperties can be decoded entirely using the tag-directed decoding
scheme. Up next, custom decoding of ClassMembers. At that point, all
definition-level decoding will be done, leaving MuIL's ASTs.
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